N'Gog is a former French youth international, having earned caps at all youth levels. However, he is also eligible to represent Cameroon, by way of his father, and has been approached by Cameroon to make the switch.

Manager Rafael Benítez hailed his scouting network after his signing, saying that "They are finding players like N'Gog, players with quality who are not a high price".[5] N'Gog played his first game for Liverpool in a friendly game against Villarreal on 30 July 2008 and scored his first goal one game later against Rangers in another pre-season friendly. He then followed this up by scoring Liverpool's final goal in their next pre-season friendly, a 4–1 win over Norwegian side Vålerenga IF. He made his Premier League debut against Aston Villa, coming on as a substitute for Fernando Torres. N'Gog scored his first goal for Liverpool in their 3–1 victory over PSV Eindhoven in the group stages of the UEFA Champions League.[6] N'Gog made his first start for Liverpool on 7 February 2009 against Portsmouth, and scored his first league goal on 3 March 2009 against Sunderland in a 2–0 win at Anfield.[7] N'Gog scored his second league goal for the club on 11 April 2009 against Blackburn Rovers at Anfield in an emphatic 4–0 win.[8]

N'Gog scored his first goal of the new season with the final goal in a 4–0 home win against Stoke City.[9] This was his fourth career goal for Liverpool, three of which came in league matches. The following month, he then went on to score the winning goal in a third-round League Cup game, to win 1–0 away against Leeds United. On 25 October, in a crucial game against Manchester United, he came on for goalscorer Fernando Torres at Anfield, scoring a goal in the 96th minute in the 2–0 win to seal an important victory for Liverpool.[10] After the crunch tie, N'Gog said scoring the goal was "a fantastic moment for me and the team and I would say it's my best moment in football." He scored his fourth goal of the season on 9 November against Birmingham City before winning a controversial penalty[11] that gave Liverpool a draw after Steven Gerrard converted from the spot.[12] N'Gog then scored Liverpool's only goal in their next match away to Debrecen in the Champions League.[13] Rafa Benitez showed his faith in N'Gog by handing him a start against Everton in the Merseyside derby; this match finished 2–0 to Liverpool.[14] He scored his fourth league goal (six in total of the 2009–10 season) in a 2–1 victory over Wigan Athletic.[15] N'Gog's goal nine minutes from time eventually broke the resistance of a determined Unirea Urziceni at Anfield in the Europa League.[16]

On 29 July, N'Gog scored twice in Roy Hodgson's first competitive match as manager in Liverpool's UEFA Europa League third qualifying round first leg 2–0 win over Macedonian side FK Rabotnički.[17] In the second leg of the tie played at Anfield, N'Gog scored the opening goal via a header and also won a penalty which Steven Gerrard scored, as Liverpool ran out 4–0 aggregate winners.[18]

N'Gog then scored again in Liverpool's opening match of the season at home to Arsenal in a 1–1 draw by turning past Thomas Vermaelen and rifling the ball into the top corner of the net past Arsenal goalkeeper Manuel Almunia. He was enjoying the best start to a Liverpool season before he was taken out of the squad that played West Brom for Fernando Torres. After being left out of the game against Birmingham City N'Gog shot to fire again in the Europa League, scoring two in Liverpool's 4–1 victory over Steaua Bucharest.[19] He scored an extra-time equaliser in the League Cup defeat to Northampton Town to take the tie to penalties, but missed his penalty in the shootout,[20] and played the remaining 35 minutes in the Reds' frustrating 2–2 draw against Sunderland at Anfield.[21] On 3 October, he replaced the injured Fernando Torres after just 10 minutes in their 2–1 home loss to Blackpool.[22] On 24 October, he replaced Joe Cole in the 80th minute in Liverpool's 2–1 Premier League win at Anfield against Blackburn Rovers. He replaced Cole for the second time, in Liverpool's 1–0 Premier League win against Bolton Wanderers at the Reebok Stadium on 31 October. N'gog was praised by Sam Allardyce for the way in which his introduction changed the game for Liverpool, indirectly leading to their late winner. He continued to be an impact sub from the bench, following the departure of Fernando Torres, and the signings of Luis Suárez and Andy Carroll and often finding himself in the starting line-up. He ended his Liverpool career at the end of the 2010–11 season with a record of 19 goals in 94 games.

N'Gog has played for France at many youth levels. He scored one goal in a 2–0 victory over England U19s in 2007.[citation needed]

In November 2009, David N'Gog was approached to play for Cameroon, despite having already represented France at youth levels. The then Liverpool striker said: "I have had contact with the Cameroonian federation. Now I need some time to think about things. I'm very flattered."[30] In February 2010, with Cameroon preparing for the 2010 FIFA World Cup, Cameroon's Minister of Sports, Michel Zoah, met with N'Gog's father on two occasions to try and convince the player to switch allegiances.[3] A few months later, in May, N'Gog was not included in Cameroon's preliminary squad for the World Cup. When asked why, Cameroon coach Paul Le Guen said that N'Gog had insisted on a starting spot with the Indomitable Lions.[31] However, N'Gog refuted the claims.[3]